1. Field of the Invention
The present invention generally relates to pulse tube cryocoolers, and more particularly to a 4-valve pulse tube cryocooler.
2. Description of the Related Art
Conventionally, a pulse tube cryocooler is used to cool an apparatus that requires a cryogenic (or very low temperature) environment, such as a Magnetic Resonance Imaging (MRI) system.
The pulse tube cryocooler repeats an operation of flowing a coolant gas (for example, helium gas) that has been compressed by a compressor to a regenerator and a pulse tube, as a working fluid, and an operation of recovering the working fluid from the pulse tube and the regenerator to the compressor, in order to form a cryogenic state at lower-temperature ends of the regenerator and the pulse tube. In addition, it is possible to absorb heat from a cooling target by thermally contacting the cooling target to the low-temperature ends of the regenerator and the pulse tube.
A 4-valve pulse tube cryocooler has a high cooling efficiency, and there are high expectations to apply the 4-valve pulse tube cryocooler in various fields.
FIG. 1 is a diagram illustrating a general structure of an example of a conventional single-stage 4-valve pulse tube cryocooler, such as that proposed in a Japanese Laid-Open Patent Publication No. 2000-18742, for example. A single-stage 4-valve pulse tube cryocooler 10 illustrated in FIG. 1 includes a compressor 12, a regenerator 40 having a high-temperature end 42 and a low-temperature end 44, and a pulse tube 50 having a high-temperature end 52 and a low-temperature end 54. The low-temperature end 44 of the regenerator 40 and the low-temperature end 54 of the pulse tube 50 are connected via a pipe 56.
Each of a high-pressure (or supply) end and a low-pressure (or recovery) end of a coolant channel of the compressor 12 branches into two channels. One channel branching from the high-pressure end of the coolant channel of the compressor 12 is connected to the high-temperature end 42 of the regenerator 40 via a first high-pressure pipe 15A having a on-off valve V1 provided thereon and a common pipe 20. In addition, the other channel branching from the high-pressure end of the coolant channel of the compressor 12 is connected to the high-temperature end 52 of the pulse tube 50 via a second high-pressure pipe 25A having a on-off valve V2 provided thereon and a common pipe 30.
Similarly, one channel branching from the low-pressure end of the coolant channel of the compressor 12 is connected to the high-temperature end 42 of the regenerator 40 via a first low-pressure pipe 15B having a on-off valve V3 provided thereon and the common pipe 20. In addition, the other channel branching from the low-pressure end of the coolant channel of the compressor 12 is connected to the high-temperature end 52 of the pulse tube 50 via a second low-pressure pipe 25B having a on-off valve V4 provided thereon and the common pipe 30. A flow control valve 60, such as an orifice, is connected to the common pipe 30.
According to the 4-valve pulse tube cryocooler having the structure described above, when the on-off valve V2 is opened in a process of supplying a high-pressure coolant gas, the coolant gas enters the pulse tube 50 via the second high-pressure pipe 25A and the common pipe 30. In addition, when the on-off valve V1 is opened, the coolant gas from the compressor 12 passes through the first high-pressure pipe 15A and the common pipe 20, and enters the regenerator 40 and further reaches the pulse tube 50. On the other hand, when the on-off valve V4 is opened in a process of recovering a low-pressure coolant gas, the coolant gas from the high-temperature end 52 of the pulse tube 50 passes through the common pipe 30 and the second low-pressure pipe 25B, and is recovered by the compressor 12. Further, when the on-off valve V3 is opened, the coolant gas from the low-temperature end 54 of the pulse tube 50 passes through the pipe 56 and the regenerator 40, and is recovered by the compressor 12 via the common pipe 20 and the first low-pressure pipe 15B.
However, during operation of the 4-valve pulse tube cryocooler 10, there is a problem in that a secondary flow of the coolant gas occurs to circulate in a closed loop indicated by an arrow L in FIG. 1, due to an unbalance of the coolant gas flows between the coolant gas supply process and the coolant gas recovery process. For example, the closed loop includes the on-off valve V2, the flow control valve 60, the pulse tube 50, the pipe 56, the regenerator 40, the common pipe 20, and the on-off valve V1. The secondary flow of the coolant gas is unidirectional and causes heat loss. For this reason, the cooling efficiency of the pulse tube cryocooler 10 greatly deteriorates when the secondary flow occurs.
The main cause of the secondary flow is a resistance, formed by the flow control valve 60, with respect to a bidirectional flow of the coolant gas, that causes the unbalance of the high-pressure coolant gas flow during the coolant gas supply process and low-pressure coolant gas flow during the coolant gas recovery process. For example, the amount of high-pressure coolant gas flowing downwards through the flow control valve 60 in FIG. 1 during the coolant gas supply process may be large compared to the amount of low-pressure coolant gas flowing upwards through the flow control valve 60 in FIG. 1. In such a case, the unbalance between the amounts of coolant gas flowing below and above the flow control valve 60 in FIG. 1 easily generates the secondary flow indicated by the arrow L.
FIG. 2 is a diagram illustrating a general structure of another example of the conventional single-stage 4-valve pulse tube cryocooler that has been proposed to suppress the secondary flow described above. In FIG. 2, those parts that are the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted.
Compared to the pulse tube cryocooler 10 illustrated in FIG. 1, a single-stage 4-valve pulse tube cryocooler 10′ illustrated in FIG. 2 does not have a flow control valve 60 in the common pipe 30. Instead, the pulse tube cryocooler 10′ has first and second flow control valves 60a and 60b. The first flow control valve 60a is provided on the second high-pressure pipe 25A, on a downstream side (that is, lower side in FIG. 2) of the on-off valve V2. The second flow control valve 60b is provided on the second low-pressure pipe 25B on an upstream side (that is, lower side in FIG. 2) of the on-off valve V4.
In the coolant gas supply process, a portion of the coolant gas from the compressor 12 flows to the pulse tube 50 via the second high-pressure pipe 25A in which the first flow control valve 60a is provided and the common pipe 30. On the other hand, in the coolant gas recovery process, a portion of the coolant gas from the pulse tube 50 flows to the compressor 12 via the common pipe 30 and the second low-pressure, pipe 25B in which the second flow control valve 325B is provided. Hence, by appropriately controlling the first and second flow control valves 60a and 60b, it becomes possible to independently control the amount of high-pressure coolant gas from the high-temperature end 52 of the pulse tube 50 to the pulse tube 50 and the amount of low-pressure coolant gas exhausted from the high-temperature end 52 of the pulse tube 50. Accordingly, the structure of the pulse tube cryocooler 10′ may suppress the secondary flow circulating in the closed loop described above.